Maximizing the yield of highly-valued petroleum products from crude oil requires that refiners convert relatively high-boiling materials such as petroleum residue, better known as resid, to more valuable, lower boiling products. One method for converting resid to more valuable products involves hydrotreating the resid and fractionating the products using a resid hydrotreating unit, or RHU.
Refiners often fractionate hydrotreated resid by atmospheric and/or vacuum distillation. The distillation tower bottoms produced in these units are known as RHU bottoms, hydrotreated resid bottoms fractions or RHU resid. Unlike most lower-boiling hydrotreated materials removed by distillation, RHU bottoms require additional processing to remove materials that foul or poison downstream processing units.
Non-reactive carbonaceous solids are one particularly undesirable component of RHU bottoms. These generally non-reactive solids are known to foul process piping, vessels and valves as they precipitate from RHU bottoms.
RHU bottoms also contain relatively high concentrations of RAMS and of metals such as vanadium and nickel. These metals are particularly undesirable RHU bottom components because they are known to poison various catalysts used in downstream processing equipment such as fluidized catalytic cracking units.
Efficient processing of RHU bottoms preferably requires fractionating the bottoms and reducing the concentration of RAMS carbon, metals and heteroatoms such as nitrogen and sulfur in the lighter bottoms fractions. This permits the lighter, higher quality fractions to be recycled for reprocessing, used as process solvents or diluents or catalytically upgraded to more valuable products. Our U.S. Pat. Nos. 4,940,529; 5,013,427; 5,124,025; 5,124,026 and 5,124,027 disclose several processes for using and upgrading RHU bottoms. Each of these patents is hereby incorporated by reference.
In our processes, two or more solvent extraction steps typically separate the RHU bottoms into two or more fractions generally characterized as asphaltenes, resins and oils. These processes typically fractionate hydrotreated RHU bottoms using a solvent extraction process which processes the bottoms through a series of near-supercritical and supercritical separators operating at a generally constant pressure and successively higher temperatures. In a typical three-stage unit, an asphaltene phase is separated from resins, oils and solvents in a first separator. A second separator separates resins from the deasphalted phase removed from the first separator, and a third separator removes solvent from an oil/solvent phase removed from the second separator.
The relatively heavy asphaltene fraction typically contains high levels of metals and RAMS carbon and can be coked or used as solid fuel. The resin fraction can be recycled to the RHU hydrotreating reactor for mixing with virgin resid feedstock, and the relatively light oil fraction can be catalytically cracked in a fluidized catalytic cracking unit (FCCU) or treated in a catalytic feed hydrotreating unit (CFHU).
While our processes provide a variety of improved methods for separating and using fractions from heavy feedstocks such as RHU bottoms, refiners desire additional processes which simultaneously can maximize the yield of the relatively light oil fraction and minimize the metal, heteroatom and RAMS carbon concentrations in the oil fraction.
It is known that solvent selection can alter the quality and yield of deasphalted oil (DAO) produced in a solvent extraction unit (SEU). Generally, when an SEU employs a relatively high molecular weight solvent, asphaltene yields are reduced and DAO yields maximized. Unfortunately, DAO produced in this manner contains relatively higher levels of RAMS carbon and metals. Conversely, lower molecular weight solvents often yield more asphaltenes and less of a higher quality DAO.
Examples 1 and 2 summarized in Tables 1 and 2 below illustrate SEU solvent selection effects. In Example 1, a three-stage solvent extraction unit employing n-butane solvent separated a hydrotreated resid into asphaltene, resin and oil fractions. The solvent to feed ratio was approximately 8:1 by volume, with the deasphalting, resin/oil and solvent recovery separators operating at about 660 psi and 291, 310 and 350 degrees Fahrenheit, respectively. In Example 2, n-pentane solvent extraction was conducted in an identical solvent extraction unit having three separators operating at about 550 psi and 363, 393 and 450 degrees Fahrenheit. Comparing Examples 1 and 2 illustrates that while the higher molecular weight pentane solvent doubled DAO yields, use of this solvent also increased the heteroatom and RAMS carbon concentration of the DAO.
TABLE 1 ______________________________________ Resid Oils Resins Asphaltenes ______________________________________ Wt % 100 17 20 63 N, wt % 0.92 0.4 0.5 1.4 S, wt % 2.78 1.42 1.88 3.24 Ramsbottom Carbon, 41 6 11 62 wt % Ni, ppm 70 1 2 131 V, ppm 140 &lt;2 &lt;2 284 ______________________________________
TABLE 2 ______________________________________ Resid Oils Resins Asphaltenes ______________________________________ Wt % 100 34 23 43 N, wt % 0.92 0.53 0.98 -- S, wt % 2.78 1.80 2.22 2.99 Ramsbottom Carbon, 41 11 23 70 wt % Ni, ppm 70 1 4 192 V, ppm 140 &lt;2 2 329 ______________________________________
Other examples of solvent selection effects can be found in the paper entitled "How Solvent Selection Effects Extraction Performance", presented in March, 1986 at the National Petroleum Refiners Association Annual Meeting, Paper No. AM-86-36. The examples presented in that paper as well as Examples 1 and 2 above suggest that an improved deasphalting process requires more than solvent optimization if oil yield and quality are to be simultaneous maximized.
Other workers in the field of solvent extraction have attempted to improve solvent extraction processes generally by increasing the number of solvent extraction steps or by recycling various process streams within the solvent extraction unit. For example, U.S. Pat. No. 4,305,814 discloses the use of up to five individual separation steps to further differentiate the components of a hydrocarbonaceous feedstock, while U.S. Pat. Nos. 4,239,616 and 4,290,880 seek to improve SEU performance by recycling a portion of the asphaltene or resin fraction through the same or subsequent separation zones.
While the SEU modifications discussed above may improve SEU performance in some cases, the modified processes are complex, not easily changed as operational requirements vary, or seem unlikely to produce significant improvements in DAO quality without decreasing DAO yields and/or increasing asphaltene yields. Therefore, refiners continue to desire a resid hydrotreating process which includes a deasphalting process capable of minimizing RHU bottoms asphaltene production while at the same time providing an oil fraction having relatively low concentrations of RAMS carbon, metals and/or heteroatoms.